The invention relates generally to a color electronic reprographic printing system, and more particularly concerns a method and apparatus for controlling the movement of a sheet to which is applied a plurality of developed images transferred thereto and the movement of a sheet gripper to prevent the image-bearing surface of the sheet from touching stationary surfaces in the printing system while the sheet is moving in a recirculating path.
The marking engine of an electronic reprographic printing system is frequently an electrophotographic printing machine. In an electrophotographic printing machine, a photoconductive member is charged to a substantially uniform potential to sensitize the surface thereof. The charged portion of the photoconductive member is thereafter selectively exposed. Exposure of the charged photoconductive member dissipates the charge thereon in the irradiated areas. This records an electrostatic latent image on the photoconductive member corresponding to the informational areas contained within the original document being reproduced. After the electrostatic latent image is recorded on the photoconductive member, the latent image on the photoconductive member which is subsequently transferred to a copy sheet. The copy sheet is heated to permanently affix the toner image thereto in image configuration.
Multi-color electrophotographic printing is substantially identical to the foregoing process of black and white printing. However, rather than forming a single latent image on the photoconductive surface, successive latent images corresponding to different colors are recorded thereon. Each single color electrostatic latent image is developed with toner of a color complementary thereto. This process is repeated a plurality of cycles for differently colored images and their respective complementarily colored toner. Each single color toner image is transferred to the copy sheet in superimposed registration with the prior toner image. This creates a multi-layered toner image on the copy sheet. Thereafter, the multi-layered toner image is permanently affixed to the copy sheet creating a color copy. The developer material may be a liquid or a powder material.
In the process of black and white printing, the copy sheet is advanced from an input tray to a path internal the electrophotographic printing machine where a toner image is transferred thereto and then to an output catch tray for subsequent removal therefrom by the machine operator. In the process of multi-color printing, the copy sheet moves from an input tray through a recirculating path internal to the printing machine where a plurality of toner images is transferred thereto and then to an output catch tray for subsequent removal. With regard to multi-color printing, a sheet gripper secured to a transport receives the copy sheet and transports it in a recirculating path enabling the plurality of different color images to be transferred thereto. The sheet gripper grips one edge of the copy sheet and moves the sheet in a recirculating path so that accurate multi-pass color registration is achieved. In this way, magenta, cyan, yellow, and black toner images are transferred to the copy sheet in registration with one another.
Some systems for transporting a copy sheet into registration with a toner image developed on a moving member accelerate the copy sheet during transfer of the toner image from the moving member to the copy sheet. Such acceleration may occur when the leading portion of the sheet is being negotiated through a nonlinear path while at the same time the trailing portion of the copy sheet is traveling through the transfer zone. An example of the above deterioration is a blurred or smeared image produced on the copy sheet.
One solution to this problem is to decouple the acceleration of the leading portion of the copy sheet while any portion of the sheet is in the transfer zone. This is done by forming a buckle in a leading portion of the sheet in a region immediately ahead of the transfer zone. Any acceleration of the leading edge of the sheet will simply decrease the size of the buckle, thus preventing the acceleration from being transmitted to the trailing portion of the sheet remaining in the transfer zone.
The buckle can be formed by advancing the leading edge of the copy sheet at a slightly lower velocity than the portion of the copy sheet in the transfer zone. For standard size sheets, such as an 81/2" by 11" sheet, the size of the buckle can be accommodated within a relatively narrow gap through which the sheet passes. This gap can be bounded by stationary surfaces that serve to control the sheet's movement by guiding its trailing edge. The surface of the sheet on which unfused toner is carried thus is not disrupted by contact with the stationary surfaces.
However, when the copy sheet is long, such as with an 11" by 17" sheet fed with the 11" edge leading, the same velocity differential between the sheet leading edge and the portion of the sheet in the transfer zone used for smaller sheets produces a correspondingly larger buckle. This larger buckle can cause the body portion of the sheet to contact the stationary surfaces, disrupting the unfused toner image. It is impractical to enlarge the system to allow more room for the larger buckle to clear the stationary portions--the system is volume-limited, and the placement of the stationary surfaces is critical to trailing-edge control. There is therefore a need to isolate leading edge accelerations from the portion of the sheet in the transfer zone while eliminating excessive sheet buckle on long sheets.
Circulation of lightweight sheet stock through a color reprographic system as described also presents problems of contact of the unfused toner image on the body portion of the copy sheet with stationary surfaces. Lightweight, flexible sheets, such as those of less than 15 pounds weight, are susceptible to relatively large displacements from a desired path when the rear portion of the sheet is not controlled, such as when the trailing edge of the sheet leaves the transfer zone. This is because the relatively low beam stiffness of such lightweight sheets is insufficient to resist the forces imposed by electrostatic attraction of the charged sheet to stationary objects and by random air currents within the reprographic system. There is therefore a need to control the movement of such lightweight copy sheets when the rear portion of the sheet is not controlled.